The present invention relates generally to carbonylation processes and specifically to a method of sequestering entrained or volatile group VIII metal catalysts in a carbonylation apparatus.
Carbonylation processes are well known in the art. Of particular commercial significance are processes for the carbonylation of methanol to make acetic acid and processes for the carbonylation of methyl acetate to make acetic anhydride. See Applied Homogeneous Catalyst With Organometallic Compounds, Cornils et al., Ed. (Bench Edition)(Wylie, Weinheim, Federal Republic of Germany 2000), Chapter 2, Parts 2.1.2 and following, pp. 104-137.
To make acetic acid, one method of choice involves carbonylating methanol in a homogeneous reaction medium wherein rhodium is utilized as a catalyst. This method is generally referred to in the art as the Monsanto process and was developed in the 1970""s. A particularly preferred method is taught in U.S. Pat. No. 5,144,068 to Smith et al. In this so called xe2x80x9clow waterxe2x80x9d process an alcohol such as methanol is reacted with carbon monoxide in a liquid reaction medium containing a rhodium catalyst stabilized with an iodide salt, especially lithium iodide along with alkyl iodide such as methyl iodide and alkyl acetate such as methyl acetate in specified proportions. With a finite concentration of water in the reaction medium, the product is the carboxylic acid instead of, for example, the anhydride. The reaction system of the ""068 patent not only provides an acid product of unusually low water content at unexpectedly favorable rates, but also exhibits unexpectedly high catalyst stability. That is, the catalyst is resistant to catalyst precipitation out of the reaction medium.
Another method of choice for carbonylating methanol involves utilizing a homogeneous iridium catalyst in the reactor. There is disclosed, for example, in U.S. Pat. No. 5,883,295, to Sunley et al. a process for the production of acetic acid comprising carbonylating with carbon monoxide methanol and/or a reactive derivative thereof, in the substantial absence of a metal promoter and/or ionic iodide co-promoter in a carbonylation reactor containing a liquid reaction composition comprising an iridium carbonylation catalyst, methyl iodide co-catalyst, water, acetic acid, and methyl acetate wherein there is maintained in the liquid reaction composition: (a) water at a concentration of less than 5% by weight; (b) methyl iodide in a concentration of greater than 12% by weight and (c) in the carbonylation reactor a total pressure of less than 50 bar. See also U.S. Pat. No. 5,877,348 to Ditzel et al. and U.S. Pat. No. 5,877,347 also to Ditzel et al.
One drawback of the iridium catalyzed homogeneous system in particular is the tendency of the catalyst to form volatile species which leads to catalyst loss. See for example U.S. Pat. No. 5,942,460 to Garland et al. at Col. 4, lines 3 and following as well as U.S. Pat. No. 5,932,764 to Morris et al. at Col. 3, line 1 and following wherein it is stated:
. . . Preferably to prevent a significant increase in the volatility of the iridium catalyst and/or optional promoter the amount of carbon monoxide in the second liquid composition withdrawn from the second reaction zone should not be reduced too low, typically to maintain at least 20% by volume of the dissolved and/or entrained gases therein . . . .
As will be appreciated by one of skill in the art, there is always incentive for improvement to existing processes, for example the precipitation in the homogeneous rhodium system is constantly subject to improvement, whereas the volatility problem in the iridium system is constantly addressed. One method proposed and worked on extensively was to introduce a supported catalyst into carbonylation systems to avoid stability/volatility/and precipitation problems. For example, there is disclosed in U.S. Pat. No. 5,466,874 to Scates et al. a polymeric carbonylation catalyst system useful for the carbonylation of methanol including a polymer support containing pendant pyrrolidone groups which support a rhodium species. See also U.S. Pat. No. 5,281,359 to Scates et al. as well as U.S. Pat. No. 5,334,755 to Yoneda et al. and U.S. Pat. No. 5,364,963 to Minami et al.
So also, U.S. Pat. No. 5,155,261 to Marston et al. discloses an improved Monsanto type process for acetic acid preparation and a heterogeneous supported catalyst for accomplishing the same. The method comprises reacting methanol with carbon monoxide under a pressure of about 65 to 80 bar in a temperature of 170-200xc2x0 C. in the presence of methyl iodide and the catalyst comprising an insoluble polymer having pendant free base, N-oxide, or quaternized pyridine groups supporting a rhodium species loaded to less than 10 weight percent (expressed as metal) of the polymer component.
WIPO Publication WO 98/57918 discloses a process for the production of acetic acid utilizing a vinyl pyridine supported Group VIII metal catalyst in a typical embodiment, about 9 percent by weight of vinyl pyridine is charged to the carbonylation reactor. See Example 1, p. 10.
Various supports have also been specifically suggested for supporting iridium catalyst. There is disclosed in U.S. Pat. No. 5,892,110 to Ramprasad et al. a process for producing acetic anhydride by the reaction of methyl acetate, carbon monoxide, and hydrogen at elevated temperatures and pressures in the presence of an alkyl halide and a heterogeneous bifunctional catalyst that contains an insoluble polymer having pendant quaternized phosphine groups some of which phosphine groups are ionically bonded to anionic Group VIII metal complexes, the remainder of the phosphine groups being bonded to iodide. The ""110 patent reports that in contrast to earlier processes no accelerator (promoter) is necessary to achieve the catalytic reaction and the products are easily separated from the catalyst by filtration. The catalyst can be recycled for consecutive runs without loss in activity. In general the catalysts include a polymer, such as a polymer with pendant phosphine groups and a Group VIII metal such as rhodium or iridium. See column 2, lines 55-60. See also WIPO Publication 98-33590, and U.S. Pat. No. 4,127,506 to Gray et al.
The present invention utilizes polymer substrates with nitrogen containing repeat units to sequester, entrained or volatile catalyst species. In this way, entrained rhodium will not xe2x80x9cplate outxe2x80x9d in the carbonylation apparatus and volatile iridium complexes can be recovered.
There is provided in a first aspect of the present invention a carbonylation process for carbonylating a reactant including a reactor containing a reaction mixture which includes a Group VIII metallic catalyst component and an alkyl halide, which reactor is coupled to a flasher configured to continuously receive a stream of the reaction mixture and separate it into a product stream and a recycle reaction mixture stream wherein the process is improved by contacting the product stream with a polymeric resin having nitrogen containing heterocyclic repeat units operative to sequester the Group VIII metal catalyst components present in the product stream. Particularly preferred are pyridine ring-containing resins and pyrrolidone ring-containing resins. Typically vinyl pyridine and vinyl pyrrolidone resins are used and these resins are crosslinked a degree of crosslinking of at least about 20% so they are insoluble in the reaction medium. Most preferably the carbonylatable reactants are methanol or methyl acetate and the alkyl halide present in the reaction mixture is methyl iodide. The Group VIII catalyst component is typically selected from the group consisting of iridium, rhodium, cobalt, ruthenium or mixtures thereof. The invention is particularly useful in connection with iridium catalyzed systems.
The invention is likewise particularly useful for removing trace amounts of a Group VIII initial catalyst component from a product stream. Typically, such trace amounts may be from 1 part per billion (ppb) up to about 150 ppm, based on the Group VIII metal content. More typically, the invention is advantageously employed on a product stream having a Group VIII metal content of anywhere from about 5 ppb to about 5 ppm, based on the content of Group VIII metal.
In another aspect of the invention there is provided a carbonylation system ? a reactor coupled to a flasher which, in turn, is coupled to a product purification system wherein:
(a) the reactor contains a reaction mixture including a Group VIII metal catalyst component and an alkyl halide promoter component;
(b) the flasher is adapted to continuously receive a stream of the reaction mixture and separate it into a liquid recycle stream which is returned to the reactor and a vapor product stream which is supplied to the purification system and includes an entrained or volatile Group VIII metal catalyst component;
(c) there is further provided, downstream of the flasher, means for sequestering the Group VIII metal catalyst component from the product stream comprising a polymeric substrate having nitrogen containing heterocyclic repeat units.
Typically, the polymer substrate is in granular or bead form and is most preferably crosslinked, polyvinyl pyrrolidone resin which is insoluble in the reaction medium. Typically, the beads of resin are in a fixed bed between the flasher and the purification system such that the fixed bed of particulate resin contacts the product stream when it is in vapor form.
In a typical apparatus the flasher is configured to adiabatically vaporize the stream of reaction mixture provided to it so as to produce the vapor product stream as further discussed herein.
In still yet another aspect of the present invention there is provided a method of making acetic acid by way of the carbonylation of methanol including:
(a) reacting methanol with carbon monoxide in a reactor provided with a homogeneous reaction medium comprising a catalytic metal component selected from the group consisting of soluble forms of iridium, rhodium, and mixtures thereof, optionally including a co-promoter selected from the group consisting of ruthenium, osmium, tungsten, rhenium, zinc, cadmium, indium, mercury and mixtures thereof, and further including methyl acetate, methyl iodide and water;
(b) supplying a stream of the reaction mixture to a flasher adapted to separate the stream of the reaction mixture into a liquid recycle stream and a product stream;
(c) recycling the recycle stream to the reactor;
(d) contacting the product stream with a resin selected from the group consisting of vinylpyridine resins and vinyl-pyrrolidone resins and mixtures thereof, the resin being operative to sequester the catalytic metal components present in the product stream and;
(e) purifying the product stream.